Valhalla 2300, Operation Manual

Page: 89 / 115

APPLICATION NOTE AN102
USING CURRENT AND POTENTIAL TRANSFORMERS WITH
VALHALLA SCIENTIFIC DIGITAL POWER ANALYZERS
102-1. General
Current and potential transformers (CT's and
PT's) may be used with any Valhalla Digital
Power Analyzer to increase its measurement
range. Clamp-on CT's may also be used to
simplify power analyzer connections.
102-2. Current Transformers
Current transformers (CT's) are used to extend
the current measurement capabilities of
Valhalla digital power analyzers. CT's are
available in many ratios, maximum current
ratings and isolation voltages.
There are two styles of CT's available, fixed
and clamp-on. Fixed CT's are generally used
for permanent test set-ups. The conductor
being measured must be disconnected, passed
through the center of the CT, and reconnected.
Clamp-on CT's are easier to use because they
are just clamped around the conductor. The
conductor need not be disconnected when
using a clamp-on CT.
CT's are also used for isolating the power
analyzer from high voltage systems. Many
high voltage systems operate at current levels
directly measurable by the power analyzer,
but with voltages beyond the power analyzer
limits. The isolation voltage rating should be
at least 1½ times the peak input voltage.
When selecting a CT, try to use decade ratios
e.g. 10:1, 100:1, etc. Decade ratios make for
easier power analyzer measurements. The
user just mentally moves the current and
power display decimal points to the right the
appropriate number of places. Of course,
non-decade ratios may also be used. If the
power analyzer is remotely controlled via the
IEEE interface, the controller can apply the
CT ratio to the measurements automatically.
The maximum current rating of the CT should
be selected 1½ times the expected current to
allow for high crest-factor waveforms.
The ratio accuracy of CT's is usually about
2%. This is quite a bit wider than the
accuracy of a digital power analyzer. The
easiest way to improve this accuracy is to
measure the actual ratio and use this value
when correcting measurements. CT's are low
frequency devices, typically 50-400Hz. At
high frequencies their phase shift will cause
the power measurement to be in error.
CAUTION!
When using CT's, never open-
circuit their secondaries while
power is applied!
The CT will act like a step-up transformer and
may produce lethal voltages which can
damage the operator and/or the power
analyzer.
Figure 1 illustrates the basic method of
connecting a CT to a digital power analyzer.
Other methods are shown in Section 6.